Bird feeders designed to keep squirrels out have been used for many years. Some of these bird feeders keep squirrels out based upon their weight. Bird feeders designed to exclude squirrels have had varying degrees of success. Squirrels have been able to bypass many of the systems designed to keep them out.
There is also a need to keep large birds from feeding at some bird feeders. Large birds can consume large quantities of expensive feed and drive small birds from bird feeders. To keep some birds out while letting other birds feed at a bird feeder requires a consistent and accurate weighing mechanism. Such a mechanism should also be adjustable to allow the owner to determine the maximum size bird that will be permitted to feed at the bird feeder.
Bird feeders should also be long lasting and inexpensive. Current bird feeders are normally fabricated from readily available materials such as pipe and flat sheets of wood, plastic, glass and metal. The fabrication time required makes them expensive. A relatively short useful life is normal. Inexpensive injection molded plastic bird feeders that are long lasting are unusual or even nonexistent. Bird feeders made from wood generally have short useful lives due to water damage and problems with fasteners that hold them together. Plastic bird feeders fabricated from plastic pipe are relatively heavy and have small capacities for bird feed. Bird feeders that include glass or plastic sheet material generally include wood materials too. These feeders have all the problems of bird feeders that are all wood.
Bird feed is primarily grains. Grains can be stored for long periods of time if they have low water content. Most grains will spoil or start to grow in a few days if their water content exceeds about 15%. Many bird feeders allow water to enter the grain storage compartment. Wet grain becomes unfit for animal consumption within hours. Molds and fungus growths which develop in wet grain may be toxic to animals.
Bird feeders generally hold sufficient bird feed to last from a few hours to a few days. The time required to refill bird feeders is an important factor because refilling is required frequently. The filling time is increased by small openings for receiving bird feed and by lids or covers that are difficult to open and close.
Bird feeders are generally designed to be hung from a tree or to be supported from the bottom on a pole. They must be above the ground to protect feeding birds from predators. A few bird feeders can be hung from the top or supported from bottom. Most bird feeders are designed to be supported in only one way and can only be changed to accommodate a different support system with difficulty.